1. Field of the Invention
The invention generally relates to photovoltaic solar cells and, more particularly, to high-efficiency multi-junction photovoltaic solar cell devices in which the monolithic device uses GaInP as a thin top cell in order to improve current matching between the top and bottom cells. The current-matched device has a higher efficiency than the current-mismatched device because of a higher device current, and, in cases of low surface-recombination velocities, a higher device voltage. Specific lattice-matched material systems which also yield higher efficiencies under the current invention include Al.sub.x Ga.sub.l-x As/GaAs (x=0.3-0.4), GaAs/Ge and Ga.sub.y,In.sub.l-y P/Ga.sub.y+0.5-y As (0&lt;y&lt;0.5).
2. Description of the Prior Art
Solar cells, also known as photovoltaic cells, are semi conductors that convert electromagnetic energy, such as light or solar radiation, directly to electricity. These semiconductors are characterized by solid crystalline structures that have energy bands gaps between their valence electron bands and their conduction electron bands, so that free electrons cannot ordinarily exist or remain in these band gaps. However, when light is absorbed by the materials that characterize the photovoltaic cells, electrons that occupy low-energy states are excited and jump the band gap to unoccupied higher energy states. Thus, when electrons in the valence band of a semiconductor absorb sufficient energy from photons of solar radiation, they jump the band gap to the higher energy conduction band.
Electrons excited to higher energy states leave behind them unoccupied low-energy positions which are referred to as holes. These holes may shift from atom to atom in the crystal lattice and the holes act as charge carriers, in the valence bond, as do free electrons in the conduction band, to contribute to the crystal's conductivity. Most of the photons that are absorbed in the semiconductor produce such electron-hole pairs. These electron-hole pairs generate photocurrent and, in the presence of a built-in field, the photovoltage of the solar cells.
Electron hole pairs produced by the light would eventually recombine, and convert to heat or a photon the energy initially used to jump the band gap, unless prevented from doing so. To prevent this phenomenon, a local electric field is created in the semiconductor by doping or interfacing dissimilar materials to produce a space charge layer. The space charge layer separates the holes and electrons for use as charge carriers. Once separated, these collected hole and electron charge carriers produce a space charge that results in a voltage across the junction, which is the photovoltage. If these separated hole and charge carriers are allowed to flow through an external load, they would constitute a photocurrent.
It is well known that photon energies in excess of the threshold energy gap or band gap between the valence and conduction bands are dissipated as heat; thus they are wasted and do no useful work. Specifically, there is a fixed quantum of potential energy difference across the band gap in the semiconductor. For an electron in the lower energy valence band to be excited to jump the band gap to the higher energy conduction band, it must absorb a sufficient quantum of energy from an absorbed photon with a value at least equal to the potential energy difference across the band gap.
A semiconductor is transparent to radiation with photon energy less than the band gap. But if the electron absorbs more than the threshold quantum of energy, e.g., from a larger energy photon, it can jump the band gap. The excess of such absorbed energy over the threshold quantum required for the electron to jump the band gap results in an electron that is higher in energy than most of the other electrons in the conduction band. Electrons that have energy levels higher than the lower edge of the conduction band, i.e., the top edge of the band gap, are referred to as "hot electrons". For every electron excited out of its normal energy level, there is a corresponding "hole". Thus, for each hot electron there can be a corresponding hot hole; both are generally referred to as "hot carriers".
These hot carriers lose their excess energy to the host lattice very rapidly as heat. The process in which the hot carriers dissipate their excess energy to the host lattice and equilibrate with the lattice at ambient temperature is known as thermalization. As a result, such thermalization of hot carriers reduces the carriers in energy to the energy level at the edge of the conduction band. Such thermalization normally occurs in about 10.sup.-12 seconds with the result that, the effective photovoltage of a single band gap semiconductor is limited by the band gap.
In practice, the effect of the limitation is that the semiconductor designer must sacrifice efficiencies in one area to achieve them in another. For example, to capture as many photons from the spectrum of solar radiation as possible, the semiconductor must be designed with a small band gap so that even photons from lower energy radiation can excite electrons to jump the band gap, but, in doing so, there are at least two negative effects that must be traded.
First, the small band gap results in a low photovoltage device, and thus low power output occurs. Secondly, the photons from higher energy radiation will produce many hot carriers with much excess energy that will be lost as heat upon immediate thermalization of these hot carriers to the edge of the conduction band. On the other hand, if the semiconductor is designed with a larger band gap to increase the photovoltage and reduce energy loss caused by thermalization of hot carriers, then the photons from lower energy radiation will not be absorbed. Therefore, in designing conventional single junction solar cells, it is necessary to balance these considerations and try to design a semiconductor with an optimum band gap, realizing that in the balance, there has to be a significant loss of energy from both large and small energy photons. Materials, such as silicon with a band gap of 1.1 eV, are relatively inexpensive and are considered to be good solar energy conversion semiconductors for conventional single junction solar cells; however, the band gap of GaAs is even better. Nevertheless, a need exists for a device that can capture and use a larger range of photon energies from the solar radiation spectrum, and yet not sacrifice either photovoltage or excess energy loss to heat by thermalization of hot carriers.
It was shown several years ago that two-junction photovoltaic cells have the potential for achieving solar energy conversion efficiencies than single junction cells..sup.1 The simplest junction device is a monolithic, two-terminal, two-junction structure, wherein the two junctions are stacked vertically. The top junction is designed to absorb and convert the blue portion of the solar spectrum and the bottom junction absorbs and converts the red portion of the spectrum that is not absorbed by the top junction. To achieve maximum energy conversion efficiency: 1) the junctions must be fabricated from materials that are of high electronic quality (usually achievable for systems which are lattice matched), and 2) they must also be current matched, i.e. generate equal currents when exposed in the tandem configuration to the solar spectrum. The current matching is determined by the relative band gap energies of the two materials. FNT .sup.1 J.C.C. Fan, B.Y. Tsaur, and B.J. Palm, Proceedings of the 16th IEEE Photovoltaic Specialists Conference (IEEE, New York, 1982), p. 692.
Only a few material combinations are known that satisfy both these criteria. Strictly speaking, only the AlxGaI.xAs/GaAs system allows for both a lattice-matched and current-matched system for both space (AMO) and terrestrial (AM 1.5) applications. However, the high aluminum content makes it difficult to achieve material of high electronic quality despite good lattice matching. Other material combinations including Ga.sub.x In.sub.l-x P/Ga.sub.x+0.5 In.sub.0.5-x As and GaAs/Ge meet both criteria depending on the spectrum under consideration, but are usually subject to some loss in efficiency. For tandem solar cells which are not exactly current matched the extra current that is generated either in the top or the bottom cell is lost. Of the multiple publications which have calculated the efficiencies of two-junction, III-V-like solar cells, the only suggested method for recovering this lost current is to use a 3.or 4-terminal device (using either independent or parallel connection). Use of 3. and 4.terminal devices has been considered to be substantially less convenient, yet a necessary remedy to an otherwise unsolvable problem for material combinations which lie away from the current-matched region.
In connection with amorphous silicon solar cells both U.S. Pat. Nos. 4,272,641 and 4,271,328 teach that the current and therefore the efficiency of a series-connected multi-junction solar cell where all of the unit cells have the same optical band gap is optimum when the thicknesses of unit cells closer to the incident light surface are selected to be less than that of cells farther from the incident light surface.
The problem that this invention addresses is taught by Hanak in U.S. Pat. No. 4,272,641. He teaches that the conversion efficiency of a single junction a-Si solar cell approaches a constant when the intrinsic region thickness exceeds about 500 nm See column 4, lines 6-16). This is due to an inherent problem with the electronic quality of a-Si. If a-Si could be made with better electronic properties then cells thicker than 500 nm would yield higher efficiencies. This problem is circumvented in a multi-junction cell where all of the unit cells have a thickness less than or equal to 500 nm. Hanak specifically teaches that the top cell (Region 22, FIGS. 1 & 2) is made to have a thickness between 40 and 500 nm (see Col. 3, lines 9-11). The thickness of the bottom cell (Region 26, FIGS. 1 & 2) is then adjusted so that "the current produced by said layer is about equal to the current produced by the first active layer 2. . . " (see Col. 4, lines 1-5).
Hanak also teaches use of multiband gap, multi-junction devices as an alternative method of matching currents in a tandem cell. The reasoning is that a top cell with a high enough band gap will obviate the need for current matching by the thinning of the top cell. Hanak and Hamakawa et al. do not teach literally a combined approach of thinning the top cell in a multiband gap, tandem solar cell.
The line of reasoning outlined above does not apply to III-V materials like GaAs, InP, AlGaAs, GaInPz, or InGaAs. They have inherently excellent electronic properties, and the conversion efficiency of devices made from these materials continue to increase for thicknesses much larger than 500 nm. Furthermore, a tandem device comprised of two GaAs unit cells (the GaAs analogue of an a-Si tandem cell) would have an efficiency less than that of a single junction GaAs cell.
We argue that the physical differences between a-Si and III-V materials is a subtle cognitive barrier that precludes the obvious foresighted extension of a-Si art to the III-V materials systems. This is evident if one considers the scope and direction of III-V tandem research and development since 1981. During this time period there have appeared in the literature several analyses of the efficiency of tandem solar cells as a function of top and bottom cell band gaps (e.g. Fan et al. Proceedinqs of the 16th Photovoltaic Specialists Conference, pp. 692-701, 1982; Nell and Barnett, IEEE Trans. Electron Devices. Vol. 34, p. 257, 1987). All of these calculations assume complete absorption by the respective unit cell of light with energy greater than the band gap energy of the unit cell, i.e. they only consider optically thick unit cells. There are also reports in the literature of the efficiency of III-V tandem devices including GaIn,P. 2/GaAs (Olson et al. 20th IEEE PVSC p.777 1988), AlGaAs/GaAs (Virshup et al.20th IEEE PVSC p.441 1988) and GaAs/Ge (Timmons et al. 20th IEEE PVSC p.602 1988). All Of these devices suffer a loss in efficiency because of poor current matching. All try to compensate for this loss by increasing the band gap of the top cell. In all cases this "cure" made the overall efficiency worse for reasons that are unique to III-V materials. In all three cases, the more effective cure would be to simply reduce the thickness of the top cell to some optimum thickness. That is exactly what Olson et al. have recognized and done, and the other "equally-skilled-in-the-art" researchers have not.